The present invention relates to a cathode ray tube, in particular to a color cathode ray tube having an electron gun employing a multistage focus lens.
Color cathode ray tubes, which are used as TV picture tubes, or monitor tubes in information terminals, house an electron gun for emitting a plurality (usually three) of electron beams at one end of an evacuated envelope, a phosphor screen formed of phosphors coated on an inner surface of the evacuated envelope at the other end thereof for emitting light of a plurality (usually three) of colors, and a shadow mask which serves as a color selection electrode and is closely spaced from the phosphor screen. The electron beams emitted from the electron gun are deflected to scan the phosphor screen two-dimensionally by magnetic fields generated by a deflection yoke mounted externally of the evacuated envelope and display a desired image on the phosphor screen.
FIG. 8 shows a cross-sectional view for explaining an example of the constitution of a color cathode ray tube, and in FIG. 8, reference numeral 1 denotes a panel portion, 1a denotes a screen, 2 denotes a neck portion for housing an electron gun, 3 denotes a funnel portion for connecting the panel portion and the neck portion, 4 denotes a phosphor screen, 5 denotes a shadow mask, 6 denotes a mask frame, 7 denotes a magnetic shield, 8 denotes a mask suspension mechanism, 9 denotes an in-line type electron gun, 10 denotes a deflection yoke, 11 denotes an internal conductive coating, 12 denotes a shield cup, 13 denotes a contact spring, 14 denotes a getter and 15 denotes stem pins.
In the case of the color cathode ray tube, the evacuated envelope is composed of the panel portion 1, the neck portion 2 and the funnel portion 3, and electron beams B emitted from the electron gun 9 housed in the neck portion 2 scan the phosphor screen 4 two-dimensionally under the horizontal and vertical deflection magnetic fields produced by the deflection yoke 10.
The electron beams B are modulated in amount by modulating signals such as video signals supplied via the stem pins 15, are color-selected by the shadow mask 5 disposed immediately in front of the phosphor screen 4, and impinge upon the phosphors of the corresponding colors to reproduce a desired image.
The cathode ray tubes of this kind are provided with a multistage focus lens in the electron gun and a dynamic focusing system is widely adopted where at least one of the electrodes constituting the multistage focus lens is supplied with a voltage varying dynamically, to obtain sufficiently small beam spots over the entire phosphor screen.
FIG. 9 is a schematic for explaining one type of an electron gun employing a dynamic focusing system which is proposed in the Japanese Patent Laid-open Publication No. Hei 8-102265, and in the FIG. 9, reference character H denotes a heater and K denotes a cathode. The first grid electrode G1 and the second grid electrode G2 form electrons generated by the cathode K into a beam and the beam is projected onto the phosphor screen being focused and accelerated by the third grid electrode G3, the fourth grid electrode G4, the fifth grid electrode G5, the sixth grid electrode G6 and the seventh grid electrode (anode) G7. Z--Z shows the direction of the tube axis.
In the case of an electron gun of this type, the sixth grid electrode G6 and the seventh grid electrode G7 form a main lens. The anode voltage Eb, the highest voltage, is applied to the seventh grid electrode G7, and a pre-main focus lens is formed by the third grid electrode G3, the fourth grid electrode G4, the fifth grid electrode G5 and the sixth grid electrode G6, and a fixed voltage VG.sub.2 is applied to the fourth grid electrode G4, and a focus voltage of a fixed voltage Vf superimposed with a dynamic voltage Vd is applied to the third grid electrode G3 and the sixth grid electrode G6. The fifth grid electrode G5 is connected to the sixth grid electrode G6 with a resistor 25 incorporated within the cathode ray tube.
The fifth grid electrode G5 is supplied with the fixed voltage Vf superimposed with the dynamic voltage Vd voltage-divided by a combination of the resistor 25, a capacitance Ca between the fifth and the sixth grid electrodes G5 and G6, and a capacitance Cb between the fourth and the fifth grid electrodes G4 and G5.
FIG. 10 is a schematic for explaining an electron gun of another type employing the dynamic focus system which is proposed in the Japanese Patent Laid-open Publication No. Hei 8-102265, and the same reference numerals as utilized in FIG. 9 designate corresponding portions in FIG. 10.
In the case of the electron gun of this type, a pre-main focus lens is composed of the electrodes, from the third grid electrode G3 to the sixth grid electrode G6, and a fixed voltage VG.sub.2 is applied to the fourth grid electrode G4 and the dynamic focus voltage Vd is applied to the third grid electrode G3 and the sixth grid electrode G6.
The fifth and the sixth grid electrodes G5 and G6 are connected with each other by the resistor 25 within the tube, the sixth and the seventh grid electrodes G6 and G7 are connected with each other by the resistor 26 within the tube, and the sixth grid electrode G6 is grounded via the variable resistor Rex external to the tube.
The sixth grid electrode G6 is supplied with the anode voltage Eb voltage-divided by a combination of the resistors 26 and Rex. The fifth grid electrode G5 is supplied with the voltage applied to the sixth grid electrode G6 superposed with the dynamic voltage Vd voltage-divided by a combination of the resistor 25, a capacitance Ca between the fifth and sixth grid electrodes G5 and G6 and a capacitance Cb between the fourth and fifth grid electrodes G4 and G5.
As shown in FIG. 9 and FIG. 10, in the case of a dynamic focus type electron gun, a plurality of voltages are required for the pre-main focus electrodes. It is difficult to supply these voltages through the stem pins in view of standardization, withstand voltage characteristics and others of the stems, so that a plurality of voltages are produced with the resistors incorporated within the evacuated envelope.
In order to produce a plurality of different voltages within a cathode ray tube, it was proposed that voltage-dividing resistors are mounted on the back of the bead glass, that is, the surface on the side of the bead glass facing away from the cathodes and grid electrodes as disclosed in Japanese Patent Laid-Open Publication Hei 7-211256, and that a high-resistance material is embedded in a groove formed in the bead glass and is tapped at proper positions thereof to provide desired resistors.
However, in the case where resistors are fixed on the back of the bead glass, it is necessary to make the bead glass thinner to secure the spacing between the resistors and the inner wall of the glass neck portion 2.
FIGS. 11A and 11B are illustrative drawings for explaining an example of the constitution of a beaded electrode assembly of a conventional electron gun in which resistor elements are fixed on the back of the bead glass. FIG. 11A shows the side view of the beaded electrode assembly and FIG. 11B shows the front view of the bead glass of the same.
As shown in FIGS. 11A and 11B, the cathode K, the first grid electrode GI, the second grid electrode G2, the third grid electrode G3, the fourth grid electrode G4, the fifth grid electrode G5, the sixth grid electrode G6 and the seventh grid electrode G7 of the electron gun are fixed in the predetermined order by embedding peripheral flanges of the grid electrodes or support tabs attached thereto in a pair of bead glass 23.
Necessary spacing for suppressing the occurrence of arcing and the like is provided between the bead glass 23 and the inner wall of the neck portion 2.
In the case of an electron gun of this type, when a resistor element 25 or 26 is fixed on the back of the bead glass 23 as it is, the resistor element becomes too close to the inner wall of the neck portion 2.
In order to avoid that, it is necessary to decease the thickness of the bead glass 23. However, when the bead glass 23 is made thinner, peripheral flanges of the grid electrodes or support tabs attached thereto cannot be embedded sufficiently deep into the bead glass 23, which injures sufficient support of electrodes. It may cause such problems as to increase the noises due to the vibration of the electrodes caused by external forces on a cathode ray tube or to develop cracks in the bead glass. In the case of a cathode ray tube in which a resistor is formed by embedding a high-resistivity material in a groove provided on the bead glass, there have been problems as mentioned below. Unlike the resistor elements fabricated as separate elements from the bead glass, it is difficult to obtain accurate values of resistance, and also it requires difficult work to dispose the high-resistivity material uniformly in the groove, which naturally increases the manufacturing cost and makes the mass production difficult.